The field of this invention is that of thermally protected electrical lighting fixtures and the invention relates more particularly to recessed lighting fixtures having means which detect the presence of thermal insulation excessively blocking dissipation of heat from the recessed fixture to interrupt operation of the lamp in the fixture to prevent overheating of the fixture.
Electrical lighting fixtures to be mounted in recessed locations are typically rated to operate at selected power levels to avoid excessive build-up of heat in the recessed lamp mounting locations. The fixture ratings are determined in anticipation that the fixture will be substantially free of surrounding thermal insulation in its recessed location but, to avoid risk of overheating, thermally responsive protector means are also used to interrupt lamp operation if fixture overheating should tend to occur. In one known system, an insulation detecting type of protector device is mounted on the fixture in such a location that thermal insulation which tends to block dissipation of heat from the fixture is also adapted to block dissipation of heat from the protector. The protector device includes a thermally responsive electrical switch which is arranged in series with the fixture lamp and which is adapted to be actuated in response to heating to a selected temperature for interrupting operation of the lamp. The protector also includes an electrical resistance heater which is connected in parallel with the lamp and which is thermally coupled to the switch means. In that arrangement, the protector device is intended to normally dissipate the heat output of the heater without actuating the switch if thermal insulation surrounding the fixture does not excessively block dissipation of heat from the fixture and from the protector device. However, if thermal insulation does excessively block heat dissipation from the fixture and protector so that the fixture would tend to overheat, the heat output from the heater actuates the switch to interrupt lamp operation and prevent such overheating. However it is found that known insulation detecting types of protector devices used in recessed lighting fixtures are likely to cause excessive nuisance interruptions in lamp operation or to permit occasional operation of the fixtures at higher temperatures than would be desired. Further, it is found that, to obtain suitable operation of such protector devices, devices frequently need to be operated at relatively high power levels which add to lighting costs and further risk development of unnecessarily high heater temperatures.
It is now found that the presence of thermal insulation surrounding a recessed lighting fixture tends to cause overheating of the fixture in large part because the thermal insulation prevents the dissipation of heat by convection around the fixture. However it is found that the dissipation of heat from the known protector devices used in such recessed lighting fixtures is due to an excessive extent to conduction of heat out of the protector via the electrical leads which are used in connecting the protector heater and switch means in the lighting circuit. That is, a significant proportion of the heat output from the heater used in such known protector devices is transferred out of the devices by heat conduction along the heater and switch leads used in a protector device. In that situation, variation in the amount of thermal insulation surrounding conventional recessed lighting fixtures tends to vary fixture temperature more readily than the temperature of the switch means used in the protector device incorporated in the fixture. As a result it is found that known protector devices tend to require precise matching to the fixtures to achieve even minimal protection against fixture overheating and, even with such precise matching, are likely to result in excessive nuisance interruption of lamp operation in the fixtures during normal variation in line voltages and ambient temperatures. Frequently the protector devices used in known recessed lighting fixtures have required operation at excessively high power levels in order to achieve even marginal performance characteristics and such higher heater power levels increase lighting costs and further risk development of unnecessarily high heater temperatures.